Cutting tools, particularly those for metal-removing machining, which also include changeable cutting inserts and cutting plates, comprise a substrate or a basic body with a single-layer or multi-layer wear protection coating deposited optionally thereon. The substrate is usually made of hard metal, cermet, ceramic, steel or high-speed steel and the wear protection coating of hard substances which are applied by the CVD or PVD process. The coating with hard substances is designed to improve the cutting properties of the tool for a particular application and to reduce the Wear of the tool.
It is also known for cutting tools to be provided with a usually thin outermost layer having a decorative and/or indicative function, for example an outermost layer of TiN, ZrN, TiC, HfC or HfN which expediently differs in colour from the underlying wear protection coating. In addition to having a purely decorative purpose, such layers can also advantageously serve for wear recognition as when the tool is used they wear and the underlying layer becomes visible, so that it can be seen with the naked eye whether a tool has already been used and how intensively.
EP 1 762 638 describes a cutting insert consisting of a hard metal substrate and a multi-layer coating of hard material for protection against wear and an outermost indicative layer of ZrN which is initially applied with a thickness of <1 μm by means of PVD and is then removed again from the cutting face and the cutting edge by brushing or abrasive blasting as the layer of ZrN as such has unfavorable tribological properties which can have a disadvantageous influence on the cutting properties at the cutting edge and cutting face.
DE 10 2004 010 285 and DE 100 48 899 also describe cutting tools with an indicative covering coating of golden yellow TiN or of TiC, HfC or HfN which differ in colour from the underlying wear protection coating, and are designed to make it possible to differentiate between a used cutting tool and an unused cutting tool with the naked eye.
The breadth of colour variation of the known covering layers with a decorative and/or indicative function, which are referred to in the following as wear recognition layers, is extremely limited. The nitrides and carbides used very frequently for wear recognition layers have for example a golden yellow colour. Apart from this, some of the known wear recognition coatings have disadvantageous tribological properties which have a disadvantageous effect on the metal cutting process and the wear characteristics of the tools. Some of the known materials used for the production of wear recognition coatings are also not sufficiently stable when exposed to the high temperatures or temperature changes occurring in many metal machining processes.